The present invention relates to an image forming apparatus and method in use as, for example, an electrophotographic full-color printer or full-color copier.
As a multi-color image forming apparatus of this type, a quad-tandem color image forming apparatus that comprises a plurality of electrophotographic process units arranged opposite to the same transfer belt is known. In the quad-tandem color image forming apparatus, various controls are executed to obtain an image superposed without displacement.
For example, a rotational speed of a drive motor for a drum or a drive motor for a belt is controlled so as to make a rotational circumferential speed of photosensitive drums of plural electrophotographic process units equal to a moving circumferential speed of the transfer belt. In addition, a plurality of photosensitive drums are arranged with a predetermined gap disposed therebetween. Therefore, the image forming timing to the photosensitive drums can be shifted in time in accordance with the gap of the photosensitive drums.
Actually, however, there are various conditions such as displacement of an exposure beam, misalignment in a pitch of a photosensitive drum, slipping between a transfer belt and the roller which drives the transfer belt, variation in the circumferential speed of the transfer belt caused by the thermal expansion in the diameter of the roller which drives the transfer belt, and the like, and it is therefore difficult to obtain a superposed image having no displacement.
Thus, the color image forming apparatus comprises a sequence of solving a problem of displacement in the superposed image by utilizing the warm-up time after opening a door at the time of turning on a power supply or in a paper jam process.
This apparatus also comprises a sequence of optimizing the image density, which is the amount of adhesion of the toner, even when the characteristics are varied due to the variation in temperature or deterioration in lifetime.
A registration sensor is used for the sequence of solving the displacement in the superposed image, and a toner density sensor is used for the sequence of optimizing the image density. The registration sensor and the toner density sensor are arranged at the most downstream side in the sheet conveying direction so as to detect marks for detection of the toner density and the displacement, which are formed on the transfer belt.
The registration sensor and the toner density sensor are arranged opposite to one another in the close vicinity of the transfer belt.
Incidentally, when a sheet on which a toner image is transferred is conveyed by the transfer belt at the time of image formation, the toner flies off from the toner image since the toner image is in a non-fixed state. For this reason, the flying toner adheres to the registration sensor and the toner density sensor, and may thereby cause stains and make the detection impossible.
If the registration sensor and the toner density sensor are covered with a hood at the time of image formation, adhesion of the flying toner can be prevented. However, the gap between the toner density sensor and the transfer belt is small, i.e. about 5 mm, and thus the hood or the like cannot be used.
In the prior art, for example, the detecting surface of the registration sensor and the toner density sensor has been cleaned by a hand or by a cleaner interlocking with the opening and closing operation of a front door of the apparatus, at the time of maintaining and checking the apparatus.
However, the stains cannot be certainly removed by the cleaning, which causes deterioration in the accuracy of detection. In the cleaning process, the detecting surface of the sensor may be scarred or, on the contrary, a lump of stains may be applied thereto.